Air pollution is a serious problem especially in large cities. In the U.S. the Environmental Protection Agency has the primary responsibility for carrying out the requirements of the Clean Air Act, which specifies that air-quality standards shall be established for hazardous substances. There are also state laws and international Protocols that set standards.
Some air pollutants are formed through the action of sunlight on previously emitted reactive materials (called precursors). For example, ozone, a pollutant in smog, is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Although many types of combustion contribute to this problem, trucks and buses have been identified as a significant source of both oxides of nitrogen (NO.sub.x) and particulate matter (PM). Pollution from internal combustion engines has been significantly reduced by burning the fuel as completely as possible, by recirculating fumes and by using catalytic converters. However, standards are constantly being changed in an attempt to lower exhaust emissions. Current standards, for the year 2004, propose NO.sub.x emissions limits of about 2 grams per brake horse power per hour (g/bhp-hr). Meeting such standards will be difficult for spark ignited (SI) engines and even more difficult for diesel engines.
In trying to meet such standards alternative fuels such as Methanol and Ethanol have been tried. Dimethyl ether, CH.sub.3 --O--CH.sub.3 hereinafter DME, is currently used as a propellant for spray cans. DME was adopted for this use as a replacement for chlorofluorcarbons. Recent studies have shown that DME is a promising new alternative fuel for compression-ignition (diesel) engines. Ultra-low exhaust emissions have been demonstrated with totally smoke free operation. Energy efficiency is equal to or better than conventional diesel engines. Exhaust gas reactivity is very low; and engine combustion noise is similar to gasoline engines. In addition, preliminary economical studies indicate that DME, when produced directly from natural gas, can be cost competitive with diesel fuel on an equal energy basis. Reference is made to U.S. Pat. No. 5,485,818 that discloses a Dimethyl Ether powered compression ignition internal combustion engine. U.S. Pat. No. 5,485,818 is hereby included by reference as a part of this disclosure.
However, conventional fuel injection equipment is not suitable for use with DME because of unacceptable internal leakage and the injection pumping rates are not appropriate. DME must be pressurized to about five bar to keep it in a liquid state under ambient conditions. At the elevated temperatures present on an internal combustion engine higher pressure (12-30 bar) is required to maintain DME in a liquid state.
The viscosity of DME is about 10% of diesel fuel and is extremely low compared to most other fuels. Diesel fuel has a kinematic viscosity at 20 degree Centigrade of 2.5-3.0 as compared to Methanol 0.75, Gasoline 0.6 and Dimethyl ether of 0.25.
The energy density of DME, although higher than the alternative fuels Methanol (CH.sub.3 OH) and Ethanol (CH.sub.3 --CH.sub.2 --OH). is much lower than conventional Diesel Fuel. Because DME has both lower density and heating values then conventional diesel fuel, volumetric flow rates of 1.7 times diesel fuel rates are necessary to achieve the same output. For example, in a six cylinder engine having an engine rate of 188 kw at 2,000 rpm, and an injection duration limit of 50 crank degrees, 250 cubic millimeters of DME must be delivered in 4 milliseconds. To accommodate this increased volume the fuel injector must have larger orifice openings.
A fuel's Cetane number, which is a measure of the fuel's ability to auto-ignite, has an important influence on diesel combustion and is a meaningful indicator of a fuel's value for diesel engines. Fuels with a high Cetane number will ignite quicker and thus will have a short ignition delay. This lowers premixed burning of the fuel, which in turn lowers NO.sub.x and noise emissions. DME has a higher Cetane number than Diesel Fuel and thus it will ignite quicker and will have a relatively short ignition delay. By throttling the amount of fuel injected during the initial portion of the injection cycle the quantity of fuel in the combustion chamber when ignition occurs is diminished. This significantly lowers NO.sub.x and noise emissions. The mechanism for throttling the fuel injected during the initial portion of the injection cycle should be time dependent such that it can be coordinated with ignition delay that is also time dependent.
Also, the vapor pressure of DME is higher than most other fuels. At 38.degree. Centigrade, the vapor pressure of DME is 8 bar as compared to 0.0069 bar and 0.35 bar respectively for Diesel fuel and Methanol. Thus DME is a gas at atmospheric conditions, but becomes liquid under modest pressure (5 bar). Therefore, in order to inject liquid fuel into the engine, the entire fuel system from tank to injection tip, must be pressurized.
The bulk modulus of DME, which is a measure of its compressibility, varies greatly with both pressure and temperature. This attribute of DME renders accurate fuel metering with jerk pump type systems extremely difficult or impossible.
The kinematic viscosity of DME at 20.degree. Centigrade is 0.25 as compared to 2.5-3.0 for diesel fuel. This relatively low viscosity of DME causes high internal leakage within the supply pump, solenoid valves and fuel injector. Thus, standard fuel storage and delivery systems are not suitable for DME.
Internal combustion engines and especially Diesel engines represent large capital investments and have long useful lives. The current process for producing DME would result in a price that would render it unacceptable as an alternative fuel. A new less costly manufacturing method has been developed to produce "raw DME" that is a form of DME that includes small amounts of water and Methanol. Large capital investments would be required to build the necessary facilities to produce raw DME at volumes that would meet its demand as an alternative fuel. Even greater capital investments would be required to provide the necessary refueling system. Large capital investments of this magnitude are unlikely to be made if the alternative fuel can only be used in newly produced special designed engines. Thus, it is likely that DME will be initially used only in urban areas where ozone and particulate matter exceed the congressionally mandated standards. A low volume niche market such as this will not justify major engine design changes or very high cost components. Thus, a very important consideration for DME as an alternative fuel is whether economical field conversions can be made to existing engines to enable them to use DME as an alternative fuel.
An early design concept for the use of DME employed liquid propane type fuel storage and delivery system, an axial piston type pump, an electronic pressure regulator valve to regulate injection pressures, a common rail which functioned as an energy storage device, a spool type 2-way solenoid valve and a conventional diesel injector nozzle. This system was tested as a computer model. This test uncovered several serious design flaws. Leakage around the closed solenoid spool valve resulted in small but uncontrolled injections. Leakage around the injection nozzle guide caused high fuel flow demand. Pump volumetric efficiency was poor and necessitated the use of a very large displacement pump. Thus, the overall system efficiency was very low which caused high fuel temperatures and necessitated the use of a large fuel cooler to control fuel viscosity & vapor pressure.
For the foregoing reasons there is a need for a DME storage and delivery system for internal combustion engines that will enable the favorable emission properties of DME to be exploited in new and existing engines.
For these reasons, there is a need for a fuel storage and delivery system that will enable internal combustion engines to be powered with DME fuel in a broad range of environmental conditions. The new and improved fuel storage and delivery system must also permit existing internal combustion engines to be economically converted in the field to be fueled by DME.